1. Field of the Invention
The present invention relates to systems and methods for optical pulse stretching.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus can include an illumination system (illuminator) configured to condition a radiation beam (e.g., DUV or EUV radiation). The illumination system can include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. The illumination system receives a radiation beam from a radiation source. In some systems, a radiation beam passes from the radiation source to the illumination system with the aid of a beam delivery system that can include, for example, suitable directing mirrors and/or a beam expander. The radiation source and illumination system, together with a beam delivery system, if required, may be referred to as a radiation system.
A lithographic apparatus can include large expensive lens elements that are difficult to fabricate. Typically, an excimer laser is used to supply the lithographic apparatus with radiation in the form of pulses. The expensive lens elements are subject to degradation resulting from billions of these high intensity ultraviolet pulses. Optical damage is known to increase with increasing irradiance (i.e., light power (energy/time) per cm2 or mJ/ns/cm2) of the pulses from the laser. The typical pulse length from these lasers is about 20 ns, so a 5 mJ laser pulse would have a pulse power of about 0.25 mJ/ns (0.25 MW). Increasing the pulse energy to 10 mJ without changing the pulse duration would result in a doubling of the power of the pulses to about 0.5 mJ/ns that could significantly shorten the usable lifetime of the lens elements.
Pulse stretching devices have been used to avoid potential optical damage by substantially increasing the pulse length (e.g., from about 20 ns to more than 50 ns) providing a reduction in the rate of optics degradation. A pulse stretching device (pulse stretcher) increases the temporal pulse length of a laser by creating copies of a laser pulse and separating them in time by an optical delay. In lithography, pulse stretchers are mainly used to increase the lifetime of optics. In addition, increasing temporal delay helps to reduce speckle. Speckle is the optical interference between beams due to temporal and spatial coherence. Superimposing portions of a beam with different time delays reduces coherence and speckle. A pulse stretcher is typically located just after the laser, or in a beam delivery system. Further information regarding pulse stretchers can be found in U.S. Pat. No. 7,432,517 B2, which is incorporated herein by reference in its entirety.
Some pulse stretcher designs use con-focal resonators, in which a pulse (also referred to in this document as a beam) is approximately collimated upon entry and exit, but passes through an intermediate focal point inside the pulse stretcher. In an unfolded implementation, e.g., where no fold mirrors are used to redirect the beam, the intermediate focal point likely causes no damage to nearby optics as it is located away from optical surfaces. Packaging constraints for longer pulse stretching, however, may require the use of fold mirrors to bend the pulse stretcher cavity into two or more parts. The insertion of one or more fold mirrors results in an optical surface where none existed in the unfolded implementation. If a fold mirror is too near the intermediate focal point, which is likely when maximizing pulse stretcher delay and making use of available space, an unacceptably high irradiance may strike the mirror. The resulting damage or a decrease in usable lifespan of the mirror due to the high energy density upon the mirror's surface is sought to be avoided.